Page 328 - Introduction to Marine Engineering

302 Instrumentation and control

the transmitter rotor position, the two e.m.f.s will be out of balance. A
current will flow and the receiver rotor will turn until it aligns with the
transmitter. The receiver rotor movement will provide a display of the
measured variable.
An electrical device can also be used as a transmitter (Figure 15.28).
The measured variable acts on one end of a pivoted beam causing a
change in a magnetic circuit. The change in the magnetic circuit results
in a change in output current from the oscillator amplifier, and the
oscillator output current operates an electromagnet so that it produces a
negative feedback force which opposes the measured variable change,
An equilibrium position results and provides an output signal

Hydraulic
The telemotor of a hydraulically actuated steering gear is one example
of a hydraulic transmitter. A complete description of the unit and its
operation is given in Chapter 12.

Controller action

The transmitted output signal is received by the controller which must
then undertake some corrective action. There will however be various
time lags or delays occurring during first the measuring and then the
transmission of a signal indicating a change. A delay will also occur in the
action of the controller. These delays produce what is known as the
transfer function of the unit or item, that is, the relationship between the
output and input signals.
The control system is designed to maintain some output value at a
constant desired value, and a knowledge of the various lags or delays in
the system is necessary in order to achieve the desired control. The
controller must therefore rapidly compensate for these system variations
and ensure a steady output as near to the desired value as practicable.

Two-step or on-off
In this, the simplest of controller actions, two extreme positions of the
controller are possible, either on or off. If the controller were, for
example, a valve it would be either open or closed. A heating system is
considered with the control valve regulating the supply of heating steam.
The controller action and system response is shown in Figure 15.29. As
the measured value rises above its desired value the valve will close.
System lags will result in a continuing temperature rise which eventually
peaks and then falls below the desired value. The valve will then open